The success of e-commerce is strongly affected by customer review systems, since they are the most importanttrust-building device in online shopping. However, participation in these systems is biased ascertain customer groups do not provide insights from their experience with the purchased goods. Thisstudy aims to stimulate the participation in customer review systems by implementing financial incentives.By conducting an economic laboratory experiment, subjects are treated with different types of incentives,namely unconditional rebates (i.e., gifts) or conditional rebates (i.e., rebates in return) for providingcustomer reviews. Through this process, providing gifts triggers reciprocity and rebates in return ratheraddress economic motives for providing reviews. Finding no evidence for peer reciprocity, we identifyseller reciprocity, economic incentives, and altruism as the drivers for providing customer reviews. In particular,we find that conditional rebates strongly increase the quantity of customer ratings but crowd outratings that are motivated by altruism. In comparison, unconditional rebates result in a lower increase ofcustomer ratings but show the advantage of not crowding out intrinsically motivated customer ratings.